Generally, aluminum wheels have been obtained by conducting a shot step of shooting a chip of metal such as a stainless alloy onto an aluminum alloy molded by means of casting, forging or the like, and then cutting intended parts.
An aluminum wheel having been subjected to such processing is further subjected to chemical conversion treatment and then coating for imparting an anticorrosive property, a cosmetic property, and weatherability is applied to the whole area or a part of the surface thereof. Since aluminum wheels are required to have a cosmetic property at a high level and an anticorrosive property under severe usage conditions, studies about such coating to obtain superior appearance and a high anticorrosive performance have been performed.
Especially in bay areas where contact with the salt of sea water is prone to occur or in cold areas where a large amount of calcium chloride as an antifreezing agent is used, a coating film is susceptible to degradation due to salt damage, and therefore there is a problem that the appearance easily deteriorates or rust is likely to be formed due to the corrosion of the aluminum alloy. What is needed therefore is a method for forming a coating film having an anticorrosive performance at a higher level enough for coping with such problems.
Especially in recent years, chemical conversion treatment with a zirconium compound as a nonchromate treatment has been conducted from the standpoint of environmental load without conducting a chromate treatment conventionally employed in chemical conversion treatment. However, the nonchromate chemical conversion treatment with a zirconium compound may be inferior to chromate treatment in anticorrosive property and therefore anticorrosive property has been secured by a multilayer coating film including a nonchromate chemical conversion treatment film, an anticorrosive coating film, and a clear coating film.
Patent Document 1 discloses an anticorrosive coating composition containing an acrylic resin having specific physical properties, an epoxy resin, powdery silica, and crosslinked polymer particles. Moreover, there is also disclosed a thick coating composition containing an acrylic resin, a blocked isocyanate compound, powdery silica, and crosslinked polymer particles. Furthermore, there is also disclosed a method for forming a coating film using the anticorrosive coating composition and a thick coating composition.
Patent Document 2 discloses a rust preventive primer coating composition containing an acrylic resin, an epoxy resin, a blocked polyisocyanate compound, organic bentonite, and an amide-based wetting dispersant.
Patent Document 3 discloses a method for forming a coating film using a rust preventive primer coating composition containing an acrylic resin, an epoxy resin, a blocked polyisocyanate compound, organic bentonite, and an amide-based wetting dispersant. However, the degradation of a coating film due to salt damage has failed to be completely controlled by methods for forming a coating film using any of the coating compositions of the patent documents, and none of the coating compositions has exhibited satisfactory anticorrosive properties under severe usage conditions in the form of a multilayer coating film in the case of nonchromate chemical conversion treatment.